Speedo calibration for larger tires

[Xtrememachines]

I put 37" cooper discover pros on and used alfaobd to adjust to the new size. The speedo is 2mph higher than it should be. Do I need to increase the tire size to lower the speedo and if so is there a formula to calculate by how much I need to change it?

 

[jadmt]

go down in size ie if you used 37" go down to 36". while parked on a smooth cement surface measure from the center of the hub to the ground and take that number by 2. probably going to be like 35.5 or something like that. use that number.

 

[Scubaz66]

Found this link while trying to decide on paying the dealer $150 to re calibrate my speedometer should I decide to purchase a larger tire for my 2022 RAM 2500 Big Horn. I'm not looking to add a Lift Kit so only looking at tires that will fit without rubbing. Just want to fill the tire well for a better look. Original tire 275/70/18 going to a 295/70/18 Speedometer only changes 2 mph. Speedometer will show 60 mph but actual speed will be 62 mph. I don't think it's worth paying the dealer to calibrate the speedometer given most Law Enforcement allow 5 miles over the posted speed because of Radar Calibrations. Hope this helps other members

275/70 R18 vs 295/70 R18 Tire Size Comparison Table with Graphic Visualization

The difference in sizes and parameters between the 275/70 R18 tire and 295/70 R18 tire. Detailed calculations and graphics.

tire-calc.com

 

[Scvette]

I asked my selling dealer service dept if they could change my tire size as I’m also putting 37” Cooper stt pros on my truck. He informed me that it’s illegal for any dealer to do that, I think it’s BS and he just doesn’t want to do it. I’m trying to find a AEV pro snap so I can do it myself.

 

[whitexc]

I asked them to lower the TPMS threshold and they said the same to me. I told the svc writer I will deal with the light.

Sent from my semi-smart telephonic device

 

[TUBBS]

Found this link while trying to decide on paying the dealer $150 to re calibrate my speedometer should I decide to purchase a larger tire for my 2022 RAM 2500 Big Horn. I'm not looking to add a Lift Kit so only looking at tires that will fit without rubbing. Just want to fill the tire well for a better look. Original tire 275/70/18 going to a 295/70/18 Speedometer only changes 2 mph. Speedometer will show 60 mph but actual speed will be 62 mph. I don't think it's worth paying the dealer to calibrate the speedometer given most Law Enforcement allow 5 miles over the posted speed because of Radar Calibrations. Hope this helps other members

275/70 R18 vs 295/70 R18 Tire Size Comparison Table with Graphic Visualization

The difference in sizes and parameters between the 275/70 R18 tire and 295/70 R18 tire. Detailed calculations and graphics.

tire-calc.com

Just curouis where is this dealer who can recalibrate speedo?

 

[firemansdiesel]

Hypertech and AEV have speedometer calibrators for our trucks, and both are $200+. My truck is 2 mph difference when I jumped to 295/70/18, small 35” from my stock 275/70/18. Was not worth it for that small increase difference. Also dealers will not go any bigger than what Ram sells. TRX tire size (35”) is the biggest they’ll flash in the programming.

 

[Scubaz66]

Just curouis where is this dealer who can recalibrate speedo?

I didn't search for a particular dealer but read somewhere a dealer can re-calibrate the speedometer. May have been in the documentation associated with the website link. I didn't have mine re-calibrated.

 

[ClawSS]

I wonder if it is more illegal to reset the speedo or represent the wrong mileage on the Odo.

 

[joshuaeb09]

Easiest way to get your calibration right, using Alfa anyways, is to gather a few numbers using something with accurate GPS speed in a steady state scenario. Capture the trucks reported speed, the actual GPS speed, and then the value you punched into Alfa. Example numbers = 79 MPH Actual, 82MPH Reported by the truck, 2270mm in Alfa. Take 79/82 = .9634 and multiply that by 2270mm. After punching in the resulting 2187mm into Alfa you should see your speedo pretty much dead on with GPS. The faster you can go the better the results (within reason). I always like to get mine or family/friends dialed in out where we have 75MPH or 80+ MPH speed limits to ensure that delta is large enough to get accurate numbers.

 

[Jqueen]

Jerry Ulhm Ram in Tampa did my re programming for free on my new 2022. I have a Carli Lift and 37” tires. The adaptive cruise would not work due to new height and speedo off do to tires. All fixed in a day no issue. Great service with no excuses of against the law or charges. Find a good dealer

 

[BadKarma]

Used Alfa to re-calibrate tire size and it gave me service esc, 4wd temporary disabled service light

 

[BadKarma]

Reverted the setting back with a saved file and service codes went away